It has been long known that by passing a particulate laden gas stream through an intense electrostatic field a large quantity of the entrained particulate matter may be ionized and stripped from the gas stream and deposited onto an oppositely charged surface. Indeed, Cottrell received his first patent (U.S. Pat. No. 895,729) directed to an electrostatic precipitator in 1908.
Briefly, there are two types of electrostatic precipitators in common use. Dry electrostatic precipitators ionize the particles which are then collected on a grounded plate. The particles are removed by a rapping mechanism which dislodges the particles from the collecting surface. A wet electrostatic precipitator operates on the same principle as the dry device. However, the dust particles collected on the grounded tube are removed therefrom by a film of wash liquid which is passed over the tube. Collector plates having various configurations have been suggested over the years, the most common being tubular and straight sided.
Electrostatic precipitators may be further classified into single stage units wherein the ionization and collection processes occur simultaneously and two-stage precipitators wherein ionization occurs in one section of the unit and collection occurs in an adjacent section of the unit.
Most conventional tube-type wet precipitators include a plurality of positively charged discharge electrode (or corona) wires circumscribed by an equal number of spaced, negatively charged tubular collecting electrodes. The tubes are fixed to an upper tube header and a lower tube header. The discharge electrodes are suspended from an upper frame extending through the tubes and are kept in alignment by a lower frame. Some designs call for the lower frame to be supported by a number of "stiff-legs" fixedly extending from the upper frame. In some embodiments, an oversized tube circumscribes the leg; the leg in this instance doubling as a discharge electrode. In this particular configuration, a plurality of corona needles normally extend from the electrode. In any event, the "stiff-legs" prevent the discharge electrodes from drifting towards the collecting tubes and disrupting the delicate coaxial symmetry necessary. Hexagonal wet precipitators, hung in a relatively similar manner, utilize welded straight segments to comprise the collecting electrodes. A cleaning fluid (usually water) is introduced above the upper header via a plurality of carefully leveled and monitored irrigation ponds and weirs to cause a liquid film to flow down the inner surfaces of the collecting electrodes.
Due to the operating characteristics of a wet electrostatic precipitator, the collecting electrodes (in addition to the other internal components) are subject to debilitating corrosion which oftentimes necessitates the repair and eventual replacement of the damaged electrodes. It goes without saying that the replacement of collecting electrodes, which are usually welded to the upper and lower tube headers, is a difficult and expensive undertaking. Moreover, tubular electrodes no matter how closely packed, require a finite deadspace between the tubes. This deadspace effectively reduces the internal particulate collecting surface area. Finally, it is difficult to maintain the proper water level in the irrigation ponds and weirs to adequately ensure a metered supply of wash fluid to the electrodes.